Nanoparticles having hollow or cracked structures are used in various fields such as catalysis, nanoreactor, chemical storage or drug delivery system for drugs, lubrication, nanoelectronics, and so on (Angew. Chem. Int. Ed. 2008, 47, 9504-9508; Angew. Chem. Int. Ed. 2004, 43, 1540-1543; Adv. Mater. 2000, 12, 808-811; Science 1999, 285, 1537-1539, 5; Nature 1997, 387, 791-793; Nat. Mater. 2005, 4, 671-675). Recently, researches for Magnetic Resonance Imaging (MRI) nano contrast agents using the nanoparticles have been actively conducted. MRI contrast agents can be classified into T2 contrast agents using iron oxide nanoparticles and T1 contrast agents using metal oxide of Mn or Gd. The iron oxide nanoparticles, which work as the T2 contrast agents, are conventionally prepared by pyrolysing FeL3 (L=CO5, NO3, acetylacetonate, and so on) precursor to directly obtain Fe3O4 of uniform sizes. Meanwhile, hollow iron oxide (Fe3O4) nanoparticles can be prepared by introducing phosphorus into the interior of iron oxide nanoparticles with replacement reaction of phosphoric acid and iron oxide, and then dissolving out the introduced phosphorus (Nano Letters, 2008, 8, 4252-4258). However, the prepared iron oxide nanoparticles include less amount of Fe, and not preferable in r2 value (relaxivity, a value representing contrast effect) and T2 contrast effect, and thus insufficient as MRI T2 contrast agents. In addition, recently, hollow Mn oxide (Mn3O4) nanoparticles are developed as T1 contrast agents having superior T1 contrast effect. The hollow nanoparticles work not only as MRI contrast agents but also as drug storages or carriers by accommodating drugs in the interior space thereof (Angew. Chem. Int. Ed. 2009, 48, 321-324). However, the Mn3O4 nanoparticles are prepared with highly toxic Mn, and thus are not preferable for humans' use, and also cannot be used as T2 contrast agents.